1. Field of the Invention
This invention relates to power supply systems for providing periodic excitation of electric discharge gas lasers.
2. Background of the Invention
High power electric discharge gas lasers such as xenon chloride, mercury bromide and carbon dioxide lasers can be operated to provide high pulsed output energy. To produce such energy the gas within the laser must be excited (pumped) by means of an intense source of (pulsed) electron excitation, either from a high voltage, self-sustained electric discharge or an electron beam sustained discharge. The excitation of the gas or gases within the laser results in the emission of light energy of the appropriate wavelength (e.g., ultraviolet, visible or infrared), which light energy can then be collimated, as is well known in the art. Due to certain practical limitations of the electron beam method of exciting the laser gases, the self-sustained electric discharge method is becoming the more popular alternative.
To provide a self-sustained electric discharge in an electric discharge gas laser (EDGL), one must first preionize the gas (low-level ionization), then avalanche the low-level ionization (avalanche ionize) to the final level appropriate for pumping the gas laser, and finally excite the gas by sufficient energy to provide the sustained discharge. Typically, the gas has been preionized by the use of low-energy ultraviolet radiation or X-rays.
In the past, single high voltage pulses having fast rise times of the order of 10's-100's nanoseconds (ns) and discharge pumping pulse durations of the order of 10's-100's ns or more have been used to avalanche ionize the gas and then to provide the required sustained discharge.
Plasma rail and magnetic switches have been used to provide such high energy, rapid rise time and long duration pulses. However, magnetic switches using self-saturating inductors (reactors) have advantages over plasma rail switches such as long life, reliability, low cost, less complexity and high repetitive rate capability. U.S. Pat. No. 4,275,317 teaches the use of saturable inductors as switches for compressing the width and sharpening the rise time of pulses from high-voltage, high-impedance pulse generators to provide the necessary excitation to EDGLs. Such saturable inductors, in conjunction with capacitance storage devices, compress the pulses from the high voltage generator until the voltage buildup (and current) reaches a level sufficient to saturate the inductor, thereby reducing its impedance to a very low value and coupling the pulses to the laser. A plurality of saturable inductor switches may be used in series to successively compress the pulse and decrease its rise time, as is discussed in the above patent.
The prior art systems using such saturable inductors for providing the electrical energy to avalanche ionize and subsequently provide a sustained discharge for lasers, while having many advantages over systems using plasma rail-type switches, still suffer from several deficiencies. First, the amplitude of the voltage pulse required to avalanche ionize the laser gas to the proper ionization level, which may be of the order of 15 to 20 kilovolts (kv), is many times greater than the amplitude of the pulse (e.g., 2-3 kv) required to provide a sustained discharge after the gas has been avalanche ionized. Also, the energy of the avalanche ionization pulse need be only a fraction of the discharge energy required for the sustained discharge pulse. When a single pulse is used to accomplish both functions, the energy level of the entire pulse must be high, thereby reducing efficiency. The impedance of the avalanche ionization pulse source will necessarily be too high for the laser during the sustained discharge. Furthermore, the output saturable inductor switch will be required to switch more power than it would if a separate low-energy, high-voltage pulse was utilized to avalanche ionize the laser gas.
In addition to the inefficiencies inherent in the use of a single pulse to provide the avalanche ionization and sustained discharge of the laser, the magnetically switched prior art power supply systems for lasers have allowed the length of the laser discharge electrodes to dictate the size of the core for the output saturable inductor switch which transfers the pulse to the laser.
The output saturable inductor switch for driving lasers of the transverse discharge type is conventionally incorporated in a transmission line geometry of the parallel plate type, as is illustrated in FIGS. 6, 7 and 8 of the above patent. Such a geometry dictates the use of a rectangular (or racetrack-shaped) core with an opening having a width slightly larger than the width of the transmission line. In most applications the required width of the core, as determined from the performance specifications for the saturable inductor, is not equal to the length of the laser electrodes. As a result, the core width is either too large or too small, thereby resulting in an excessive power loss within the core or improper switching. For example, an article entitled "Magnetic Modulators for Low-Impedance Discharge Lasers" by E. Y. Chu, G. Hoffman, H. Kent and T. Beinhardt published in the IEEE Conference Record 1982, 15th Power Modulator Symposium, pp. 37-46, describes the use of an inductor core having a width of one meter for switching a pulsed power source to a one-meter-long mercury-bromide laser. A parallel plate transmission line of approximately one meter in width was used to carry the pulses from the switch to the laser electrode. The use of a smaller core for the inductor switch would have significantly increased the overall efficiency of the laser power supply system.
The present invention solves the above problems by providing separate avalanche ionization and sustained discharge pulses and a transmission line for coupling a saturable inductor core of one size to laser electrodes of another size.